Hi from Toronto, where I'm attending the Ontario Genealogical
Society's annual conference! And this year they have their first-ever
Jewish stream, which will take place on Sunday. I'm speaking then,
which means that I was able to enjoy today.
I flew into Buffalo so got to enjoy (for about 10 minutes) the beauty of Niagara Falls.
When I arrived at the conference hotel, I met up with Israel Pickholtz of All My Foreparents and Marla Waltman of JGS Toronto (as well as a fellow Shpikov descendant). We checked out the expo hall and had lunch.
Then I went to CeCe Moore's talk on autosomal DNA.
CeCe gave a very good overview of autosomal DNA.
I'll skip the basics, but there were a few points that made an
impression on me, and I'll share here.
One thing I didn't realize before is that recombination is faster female to female, so females tend to drop out of your genome in fewest generations. Segments that stay in your genome over multiple generations are most likely from the direct paternal line. There isn't a huge difference between different ancestral lines, but that bias towards paternal ancestors is still there. This gives yet another reason to test the oldest generation possible, as an ancestor whose DNA is carried at that generation may not exist any more in your own.
She showed how much shared DNA can range for people of the same relationship within her own family. So it's difficult to know how close a match is just looking at the amount of shared DNA. And that's when you're not from an endogamous population.
She mentioned that Ancestry's Tinder algorithm removes "pile-up" regions--these are regions where an inordinate number of people match one another, so it's likely not applicable to genealogy. However, this means that some matches may be underestimated. She has seen a 10% increase in shared cM when in GedMatch. (Ancestry also doesn't factor in matches on the x chromosome.)
CeCe suggests that when looking at triangulations, and trying to find a common ancestors among matches with shared triangulated segments, you need to make sure you're not looking at a pile-up region. Perhaps you have many many people matching on the same section because it's a pile-up region, not a true recent common ancestor.
Ancestry's new algorithm is much more conservative, so some 2nd cousins may end up in 3rd-4th.
She talked a bit about endogamy and how to deal with it. She made an important point in that endogamy is really extreme and occurs over centuries. So she calls populations such as colonian Americans pedigree collapse; this impacts matches less than true endogamy--in which she includes Ashkenazic Jews, Icelanders & Finns. Endogamy has a much more extreme effect on matches. Really important to look at chromosome browser to see size of segments, not just total cM when you're looking at endogamy.
When dealing with Ashkenazi Jews, she wants want to see a segment of 20cM or better to have a chance of finding a traceable common ancestor.
Then I went to an invitation-only talk by Israel Pickholtz on endogamy.
He gave many examples of what endogamy is and why it
makes life so much more interesting dealing with endogamous DNA than
non-endogamous. Shout-out to Debbie Long of the Triangle JGS for having her endogmous DNA featured in the talk as a wonderful example of being related to most of the Pickholtz people!
Then off to my friend Yonina's for a wonderful Shabbos. And now it's Sunday and I'm about to speak (aaah!), so stay tuned for today's happenings in a future post.
Note: I'm on Twitter. Feel free to follow me (@larasgenealogy).
I flew into Buffalo so got to enjoy (for about 10 minutes) the beauty of Niagara Falls.
 |
| Niagara Falls |
Then I went to CeCe Moore's talk on autosomal DNA.
 |
| CeCe Moore at OGS |
One thing I didn't realize before is that recombination is faster female to female, so females tend to drop out of your genome in fewest generations. Segments that stay in your genome over multiple generations are most likely from the direct paternal line. There isn't a huge difference between different ancestral lines, but that bias towards paternal ancestors is still there. This gives yet another reason to test the oldest generation possible, as an ancestor whose DNA is carried at that generation may not exist any more in your own.
She showed how much shared DNA can range for people of the same relationship within her own family. So it's difficult to know how close a match is just looking at the amount of shared DNA. And that's when you're not from an endogamous population.
She mentioned that Ancestry's Tinder algorithm removes "pile-up" regions--these are regions where an inordinate number of people match one another, so it's likely not applicable to genealogy. However, this means that some matches may be underestimated. She has seen a 10% increase in shared cM when in GedMatch. (Ancestry also doesn't factor in matches on the x chromosome.)
CeCe suggests that when looking at triangulations, and trying to find a common ancestors among matches with shared triangulated segments, you need to make sure you're not looking at a pile-up region. Perhaps you have many many people matching on the same section because it's a pile-up region, not a true recent common ancestor.
Ancestry's new algorithm is much more conservative, so some 2nd cousins may end up in 3rd-4th.
She talked a bit about endogamy and how to deal with it. She made an important point in that endogamy is really extreme and occurs over centuries. So she calls populations such as colonian Americans pedigree collapse; this impacts matches less than true endogamy--in which she includes Ashkenazic Jews, Icelanders & Finns. Endogamy has a much more extreme effect on matches. Really important to look at chromosome browser to see size of segments, not just total cM when you're looking at endogamy.
When dealing with Ashkenazi Jews, she wants want to see a segment of 20cM or better to have a chance of finding a traceable common ancestor.
Then I went to an invitation-only talk by Israel Pickholtz on endogamy.
 |
| Israel Pickholtz speaking at OGS |
Then off to my friend Yonina's for a wonderful Shabbos. And now it's Sunday and I'm about to speak (aaah!), so stay tuned for today's happenings in a future post.
Note: I'm on Twitter. Feel free to follow me (@larasgenealogy).
"When dealing with Ashkenazi Jews, she wants want to see a segment of 20cM or better to have a chance of finding a traceable common ancestor." This seems a little extreme. If I had followed this advice, I wouldn't match at all with my 3rd cousin once removed. However, it may make sense for people who have tested a lot of known relatives.
ReplyDelete"Pile-up"? How they explain it, mechanistically? Is it some sort of an inversion preventing internal recombinations? Or a segment with very low informative marker content? Or an assay artifact making "all people within a lab or reagent batch" look similar even if their genotypes aren't really similar?
ReplyDeleteThe AncestryDNA algorithm for detecting and removing overly common matching regions is called "Timber", as in what one shouts when felling a tree.
ReplyDeleteRegarding "pile-up" regions, see my post here for examples:
http://ourpuzzlingpast.com/geneblog/2015/01/31/chromosome-pile-ups-in-genetic-genealogy-examples-from-23andme-and-ftdna/